OK, let's work through this together. We're starting with 843 and we're going to multiply it by 21, which means we add 843 to itself 21 times.
Step 1: 0 + 843 = 843
Step 2: 843 + 843 = 1686
Step 3: 1686 + 843 = 2529
Step 4: 2529 + 843 = 3372
Step 5: 3372 + 843 = 4215
Step 6: 4215 + 843 = 5058
Step 7: 5058 + 843 = 5901
Step 8: 5901 + 843 = 6744
Step 9: 6744 + 843 = 7587
Step 10: 7587 + 843 = 8430
Step 11: 8430 + 843 = 9273
Step 12: 9273 + 843 = 10116
Step 13: 10116 + 843 = 10959
Step 14: 10959 + 843 = 11802
Step 15: 11802 + 843 = 12645
Step 16: 12645 + 843 = 13488
Step 17: 13488 + 843 = 14331
Step 18: 14331 + 843 = 15174
Step 19: 15174 + 843 = 16017
Step 20: 16017 + 843 = 16860
Step 21: 16860 + 843 = 17703

So, 843*21 = 17703